app/cli/lib/fileDiff.ts

 // A real unified diff (not a side-by-side full-file redump) between two

 // in-memory file sets -- what `zz work --assist`'s chat transcript shows

 // for a revision, so a human reviewing a second/third draft sees what

 // actually changed instead of re-reading the whole file from scratch

 // every turn. Shells out to `git diff --no-index` against two temp

 // files rather than hand-rolling a diff algorithm: it's already

 // installed everywhere this runs, and `--color=always` gives a real

 // ANSI-colored unified diff for free.

 import { execFileSync } from "node:child_process";

 import { mkdtempSync, rmSync, writeFileSync } from "node:fs";

 import { tmpdir } from "node:os";

 import { join } from "node:path";

 export interface FileEntry {

  path: string;

  content: string;

 }

 /** One block of diff text per changed/added file (joined with blank lines); omits files whose content is identical in both sets. Empty string if nothing changed. */

 export function diffFileSets(before: FileEntry[], after: FileEntry[]): string {

  const beforeMap = new Map(before.map((f) => [f.path, f.content]));

  const blocks: string[] = [];

  for (const f of after) {

  const prev = beforeMap.get(f.path);

  if (prev === f.content) continue;

  if (prev === undefined) {

  blocks.push(`--- ${f.path} (new) ---\n${f.content}`);

  continue;

  }

  blocks.push(`--- ${f.path} ---\n${unifiedDiff(prev, f.content, f.path)}`);

  }

  return blocks.join("\n\n");

 }

 // Drops git's own "diff --git <tempdir>/a <tempdir>/b" / "index .." /

 // "--- a" / "+++ b" header lines -- the caller already prints a clean

 // "--- <real path> ---" label above this, and the temp paths those

 // headers carry are meaningless noise in a chat transcript. Keeps

 // everything from the first "@@" hunk marker onward.

 function stripHeader(diffText: string): string {

  const lines = diffText.split("\n");

  const hunkStart = lines.findIndex((l) => l.replace(/\x1b\[[0-9;]*m/g, "").startsWith("@@"));

  return (hunkStart === -1 ? lines : lines.slice(hunkStart)).join("\n");

 }

 function unifiedDiff(before: string, after: string, label: string): string {

  const dir = mkdtempSync(join(tmpdir(), "zz-diff-"));

  try {

  const a = join(dir, "a");

  const b = join(dir, "b");

  writeFileSync(a, before);

  writeFileSync(b, after);

  try {

  // git diff --no-index exits 0 only when the files are identical

  // (never reached here -- the caller already checked that) --

  // exit 1 means "real diff produced," which execFileSync treats

  // as a thrown error whose own .stdout still has the diff text.

  return stripHeader(execFileSync("git", ["diff", "--no-index", "--color=always", a, b], { encoding: "utf8" }));

  } catch (err) {

  const stdout = (err as { stdout?: string }).stdout;

  return stdout ? stripHeader(stdout) : `(couldn't compute a diff for ${label} -- showing it unchanged)`;

  }

  } finally {

  rmSync(dir, { recursive: true, force: true });

  }

 }